The invention relates to a method and apparatus for electrolytic purification of metals. An electrolysis is carried out from an impure metal anode to a pure metal cathode wherein impurities more electronegative than the metal to be purified remain at the anode terminal. Where the metal is gallium, which is used in light-emitting diode construction and has a low melting temperature, the process is carried out with the metal essentially molten in form.
The basic process of electrochemical purification of gallium uses electrolyte in conjunction with electrodes of pure and impure gallium such that a current flow transports the gallium ions from an impure supply of gallium to the pure supply. The purification of gallium by electrolysis from anode to cathode allows the removal of all metals more electronegative than gallium. Thus, for example, Ag, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Sn will not electrooxide at the gallium oxidation potential in a basic solution. These metals should remain in the impure anode gallium supply if the potential of the anode does not significantly exceed the gallium oxidation potential. This basic process of electrochemical purification of gallium is described in an article by Lysenko and Tsyb, entitled "Removal of Micro-impurities from Gallium", F. Appl. Chem., USSR 38 (3), 501 (1965).
In attempting to apply this process to the present day need to recycle scrap gallium, there are several difficulties which prohibit the application of this process for the production of semiconductor quality gallium. Impurity levels on the order of a few parts per million were about the best possible with the Lysenko process whereas present semiconductor technology requires impurity levels at or below a few parts per billion. Additionally, the phenomenon of passivation of the gallium anode occurs; that is, chemical dissolution of gallium into the solution increasing the gallium level such that eventually the gallium oxide compounds increase the resistance of the cells to an extent that current will no longer flow. This necessitates completely changing the electrolyte fluid and results in a loss of the gallium dissolved therein. Since it is economically unfeasible to throw away this gallium, a separate process must be utilized to recover the dissolved gallium from the discarded electrolyte. Therefore, the need arose for both a method and an apparatus whereby gallium could be continuously electrochemically purified without constantly changing electrolytes with a degree of purity that is suitable for present day semiconductor technology.